The present invention relates to a seat, in particular an office swivel chair comprising a foot with a center column extending upwards therefrom, wherein the center column may be variable in length and/or spring mounted, a seat carrier secured at the upper end of the center column, a seat top pivotable around a horizontal axis close to the front edge of the seat top, connected with the seat carrier, and biased by a spring arrangement with an upwards directed force, and a back rest attached to at least one upwardly extending lever which is articulately connected with the seat carrier at a first link point under the seat top, and, spaced therefrom, is articulately connected with the underside of the seat top at a second link point, wherein both link points comprise swivelling axes in parallel with the seat top swivelling axis.
A seat of the type mentioned is known from U.S. Pat. No. 4,709,962. With such a seat, the lever carries the back rest and is linked at the seat carrier and the seat top, and serves for generating a coupled movement of scat top and back rest. More specifically, the coupling is comprised such that the back rest also pivots when the seat top is pivoted, however, in a different angle than the seat top. The ratio of the pivot angles to each other is set at a predetermined value in the range between approximately 1.1.3 and 1.2.5 with seats known in the current market. With this construction of the seat, in particular, the user of the seat may take his/her position between an upright position with a raised seat top and a straight backwards directed back rest, and a reclined position with a seat top lowered in the rear area and a back rest in an angled position, thereby avoiding the situation that when changing the seat position the back rest will shift at the back of the user upwards or downwards in an unpleasant way.
A further relevant seat is known from German published application 43 12 113 C1. This document describes a seat with the seat adjustable in the inclination thereof and pivotable around a horizontal swivelling axis in the front area, and a back rest with an over proportional inclination in relation to the inclination change of the seat. Therein the seat is articulately connected with swivelling levers with one end thereof carrying the back rest, and with the other end thereof rotatably supported at the bearing support of the chair structure. The swivelling levers are spaced from the rear end of the seat and linked thereto, and the sections of the swivelling levers are rotatably supported at the bearing support, and take an obtuse angle towards the support area in relation to the sections of the swivelling levers adjacent to the link axis of the seat. Furthermore, a return spring is provided to automatically reset the seat top and the back rest from the idle position into the working position when the load is released. In order to adapt the seat to a wide range of user weights it is provided with this seat that the swivelling axis of the swivelling levers is displaceable in relation to the bearing support, and that the return spring is arranged between the bearing support and the sections of the swivelling levers rotatably supported at the bearing support, and that it is adjustable in the mounting position in relation to the swivelling bearings variable, such that the torque exerted by the spring force around the swivelling axis is variable. It is not provided that the ratio between the swivelling angle of the seat and the swivelling angle of the back may be varied with the seat.
From German published application 44 39 290 A1, a synchronous adjusting device for office chairs, seats and the like is known comprising a seat member, a back member with a back rest attached to a back carrier or integrated therein, and a stationary carrier construction to which the seat member is linked, wherein the back carrier is connected with the seat member and the carrier structure, and the seat member carries out a swivelling movement when the back inclination is enlarged. In order to keep the number of link points, and the number of the additional connecting points, low and to lower the construction effort of such a synchronous adjustment device, it is provided that the back carrier is directly pivotally connected with the seat member and with the carrier structure. In an embodiment of this adjustment device, with at least one of the swivelling connections, a shifting gate for the turning axis is provided for overlapping a displacing movement with a rotational movement around the swivelling axis. Thereby influence may be taken on the course of the synchronous movements of the scat member and the back member with a corresponding shape of the shifting gate. It is a disadvantage of this synchronous adjustment device that the course of the synchronous movements of the seat member and the back member is set by the shape of the shifting gate, and may no longer be influenced by the user of the seat. Furthermore, with this example of the seat, a synchronous adjustment is provided in such a way that when pivoting the back rest backwards, the seat top is raised in its lower area, and that vice versa when pivoting the back rest to the front, the seat top is lowered in the rear area. This kind of synchronous adjustment is considered no longer to be ergonomicly advantageous.
It is a disadvantage of all these known prior art seats that, because of the preset ratios between the swivelling angles of seat top and back rest, there is always a compromise between the different requirements of various users which occurs because of different anatomical conditions. The ratio of the swivelling angles of seat top and back rest is always preset when manufacturing the seat, and afterwards may not be changed by the user.